The present disclosure relates to a coordinate measuring device. One set of coordinate measurement devices belongs to a class of instruments that measure the coordinates of a point by sending a laser beam to the point. The laser beam may impinge directly on the point or may impinge on a retroreflector target that is in contact with the point. In either case, the instrument determines the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest. Exemplary systems for determining coordinates of a point are described by U.S. Pat. No. 4,790,651 to Brown et al. and U.S. Pat. No. 4,714,339 Lau et al.
The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more laser beams it emits. A device that is closely related to the laser tracker is the laser scanner. The laser scanner steps one or more laser beams to points on a diffuse surface. The laser tracker and laser scanner are both coordinate-measuring devices. It is common practice today to use the term laser tracker to also refer to laser scanner devices having distance- and angle-measuring capability. This broad definition of laser tracker, which includes laser scanners, is used throughout this application.
One type of laser tracker contains only an interferometer without an absolute distance meter. If an object blocks the path of the laser beam from one of these trackers, the interferometer loses its distance reference. The operator must then track the retroreflector to a known location before continuing the measurement. A way around this limitation is to put an absolute distance meter (ADM) in the tracker. The ADM can measure distance in a point-and-shoot manner. Some laser trackers contain only an ADM without an interferometer. An exemplary laser tracker of this type is described in Payne, et al U.S. Pat. No. 5,455,670. Other laser trackers typically contain both an ADM and an interferometer. An exemplary laser tracker of this type is described in Meier, et al. U.S. Pat. No. 5,764,360.
One of the main applications for laser trackers is to scan the surface features of objects to determine their geometrical characteristics. For example, an operator can determine the angle between two surfaces by scanning each of the surfaces and then fitting a geometrical plane to each. As another example, an operator can determine the center and radius of a sphere by scanning the sphere surface. Up until this time, an interferometer, rather than an ADM, has been required for the laser tracker to scan. The reason for this is that absolute distance measurements have only been possible on stationary targets. Consequently, to get full functionality with both scanning and point-and-shoot capability, laser trackers have required both an interferometer and an ADM. What is needed is an ADM that has the ability to accurately and quickly scan a moving target. This permits tracker cost to be reduced because the interferometer is no longer needed.